A brake disk has an annular rim projecting radially outward from a cylindrical cup-shaped hub and having a pair of axially opposite annular faces that lie in respective parallel planes perpendicular to the symmetry axis of the disk. Such a disk is cast of metal and is given its finished shape by rotating it about its axis on a lathe-like machine that cuts the critical hub and rim faces. Since this material-removing roughing operation scores the rim faces, such a roughed workpiece must subsequently be finished by grinding at least its rim faces.
The standard procedure for finish grinding is to clamp the hub of the roughed disk and rotate the disk about its axis while urging a grinding stone axially against each rim face. Although such a procedure should in theory produce a perfectly planar finish in the brake disk, in reality the results are less than satisfactory. Invariably the rotation axis of the disk moves a little, for instance due to worn bearings supporting the disk-holding chuck, so that the resultant surface can be somewhat wavy and nonplanar.
As described in U.S. Pat. No. 4,456,401 of Kushmuk it is known to grind the faces of a metallic workpiece, in particular of a brake disk, by rotating the disk about its axis and urging against each of its faces a grinding stone that in turn is rotated about an axis at least generally parallel to the disk-rotation axis. According to this patent the grinding stone is rotated about an axis that is parallel to the disk axis, but it is mounted on a universal joint so that it can rock somewhat with respect to its rotation axis and thereby follow the surface being ground. A spring braced across the joint urges the grinding disk into a position centered on an axis parallel to that of the disk, while permitting the above-mentioned rocking.
More specifically in the Kushmuk patent the grinding stone has a sleeve-like hub that fits over a grinding-tool drive shaft that rotates about its own axis that extends parallel to the disk axis. This hub has a ring with a universal-joint formation that engages the back of the grinding tool. The tool itself has two diametrally opposite pins extending orthogonal to the grinding-tool axis and that are engaged in slots in the sleeve-like hub. In this manner during grinding the spring presses the grinding tool against the disk face while the pins rotationally entrain the tool. The pins, however, inhibit the desired swiveling action of the universal-joint formations so that the resultant finish is not completely satisfactory. The loose fit can lead to uncontrolled vibration that produces a wavy finish in the brake disk.